1. Field of the Invention
The present invention relates to a holography electron microscope comprising a transmission electron microscope (TEM) equipped with an electron biprism.
2. Description of the Related Art
A holography electron microscope comprises a TEM having electron optics in which an electron biprism is inserted. This instrument is constructed as shown in FIG. 7. Shown in this figure are an electron gun 1, an illumination lens system 2, a specimen 3, a specimen holder 4, an objective lens 5, an auxiliary objective lens 6, an electron biprism 7, intermediate lenses 8, a projector lens 9, and an observation/recording device 10. This observation/recording device 10 is made of a fluorescent screen, a photography device, or a TV camera.
When the electron biprism 7 is not operated, the instrument functions as a normal TEM. That is, the electron beam produced by the electron gun 1 is directed onto the specimen 3 via the illumination lens system 2. The electron beam transmitted through the specimen 3 is magnified by a magnification-and-projection system comprising the objective lens 5, the auxiliary objective lens 6, the intermediate lenses 8, and the projector lens 9. Finally, the electron beam is projected as a TEM image onto the observation/recording device 10.
The structure and operation of the electron biprism 7 are next described. As shown in FIG. 8, the electron biprism 7 comprises a filament 11 and two electrodes 12 and 13 located on the opposite sides of the filament 11. In FIG. 8, the crossover position that is the position of the back focal plane of the objective lens 5 is indicated by C. The image plane of the objective lens 5 is indicated by IP.
The filament 11 is normally made of thin wire of platinum having a diameter of about 1 xcexcm. During operation, a positive voltage of hundreds of volts is applied to the filament 11. The electrodes 12 and 13 are grounded. Sometimes, none of the electrodes 12 and 13 may be mounted. In this example, it is assumed that the instrument is fitted with these electrodes 12 and 13.
When a given voltage is applied to the filament 11 to operate the electron biprism 7, the electron beam transmitted through the specimen 3 is divided into two by the filament 11. The resulting two portions of the electron beam overlap each other as shown, producing interference. Interference fringes are created in the region in which interference occurs. In FIG. 8, the width of the overlap (hereinafter referred to as the interference region) is indicated by D. The auxiliary objective lens 6 is omitted in FIG. 8.
In this case, the specimen 3 is examined as follows. One of the two portions obtained by dividing the electron beam by the filament 11 passes through a space in which the specimen does not exist (i.e., a vacuum). The other portion is transmitted through the specimen. In this way, the interference fringes created in the interference region by the two portions of the electron beam contain information about the phase shift that electrons transmitted through the specimen undergo during passage through the specimen.
Since the electron beam transmitted through the specimen contains information about an object of interest, the beam is known as an object wave. The electron beam transmitted through a vacuum is not affected at all and, therefore, can provide a reference. Hence, this beam is termed a reference wave. The interference fringes created in the interference region are known as an electron hologram. Procedures for analyzing the phase of the object wave based on the electron hologram and analyzing information about the specimen are referred to as electron holography.
The electron hologram created by the electron biprism 7 is magnified by the rear intermediate lenses 8 and projector lens 9 and focused onto the observation/recording device 10. The image plane of the objective lens 5, i.e., the object plane of the intermediate lenses 8, is made noncoincident with the position of the electron biprism 7. In particular, the image plane of the objective lens 5 is normally placed in position below the electron biprism 7.
The spacing I between the interference fringes of the electron hologram and the width D of the interference region are next described. The spacing between the interference fringes depends on the voltage applied to the filament 11 of the electron biprism 7 and on the position of the image plane IP of the objective lens 5. The spacing I between the interference fringes at the image plane IP of the objective lens 5 is given by:                     I        =                                            (                              a                +                b                            )                        ⁢            λ                                2            ⁢            a            ⁢                          xe2x80x83                        ⁢            ϕ            ⁢                          xe2x80x83                        ⁢                          V              f                                                          (        1        )            
where a is the distance from the crossover position C of FIG. 8 to the filament 11, b is the distance from the filament 11 to the image plane IP, xcex is the wavelength of electrons, Vƒis the voltage applied to the filament 11, and "PHgr" is the deflection sensitivity of the electron biprism 7. The width D of the interference region from which the thickness of the filament 11 itself of the electron biprism 7 is subtracted is given by:                     D        =                              2            ⁢            b            ⁢                          xe2x80x83                        ⁢            ϕ            ⁢                          xe2x80x83                        ⁢                          V              f                                -                                    2              ⁢                              (                                  a                  +                  b                                )                            ⁢              r                        a                                              (        2        )            
where r is the radius of the filament 11 of the electron biprism 7.
In electron holography, the spacing I between the interference fringes of the electron hologram and the value of the width D of the interference region converted onto the specimen surface are important. The spacing Is between the interference fringes converted onto the specimen surface is given by:                               I          s                =                                            (                              a                +                b                            )                        ⁢            λ                                2            ⁢            aM            ⁢                          xe2x80x83                        ⁢            ϕ            ⁢                          xe2x80x83                        ⁢                          V              f                                                          (        3        )            
The width of the interference region Ds converted onto the specimen plane is given by:                     Ds        =                                            2              ⁢              b              ⁢                              xe2x80x83                            ⁢              ϕ              ⁢                              xe2x80x83                            ⁢                              V                f                                      M                    -                                    2              ⁢                              (                                  a                  +                  b                                )                            ⁢              r                        Ma                                              (        4        )            
The magnification M of the objective lens 5 at the image plane IP is used in Eqs. (3) and (4). This magnification M is now discussed. The objective lens 5 is normally excited strongly. Its focal distance ƒ can be made sufficiently smaller than the sum of the distances (a+b) and so the magnification can be given by:                     M        =                              a            +            b                    f                                    (        5        )            
Accordingly, where the objective lens 5 is excited strongly and its focal distance ƒ is made sufficiently smaller than the sum of the distances (a+b), the interference fringe spacing Is and the interference region width Ds converted onto the specimen surface can be given by Eqs. (6) and (7), respectively, by substituting Eq. (5) into Eqs. (3) and (4).                               I          s                =                  fλ                      2            ⁢            a            ⁢                          xe2x80x83                        ⁢            ϕ            ⁢                          xe2x80x83                        ⁢                          V              f                                                          (        6        )                                Ds        =                                            2              ⁢              fb              ⁢                              xe2x80x83                            ⁢              ϕ              ⁢                              xe2x80x83                            ⁢                              V                f                                                    a              +              b                                -                                    2              ⁢              fr                        a                                              (        7        )            
An example of the calculation is now given. It is assumed that the accelerating voltage for the electron beam is 200 kV and that a=150 mm. Normally, the distance b is about 10 mm. These are practical values. In this case, the deflection sensitivity "PHgr" of the electron biprism 7 is roughly 1xc3x9710xe2x88x926 rad/V. If we assume that Vƒ=200 V,ƒ=2 mm, and r=0.3 xcexcm, then Is=0.084 nm and Ds=42 nm.
It is assumed that the resolution of a recorder set in the observation/recording device 10 is 20 xcexcm. To record an electron hologram, the final image magnification (total magnification) needs to be in excess of 250,000 times. The width of the interference region of the electron hologram recorded when the final image magnification (total magnification) is 250,000 times is as narrow as 10 mm. To secure a sufficient field of view by increasing the width of the interference region, the voltage Vƒ applied to the filament 11 may be increased. However, as can be easily seen from Eq. (1), if the voltage Vƒapplied to the filament 11 is increased, the spacing I between the interference fringes at the image plane IP of the objective lens 5 narrows. Therefore, if it is attempted to increase the effective field of view at the position of the observation/recording device 10, the magnification will be shifted to higher values.
A method of solving this problem consists of exciting the objective lens 5 weakly, bringing the position of the image plane IP close to the objective lens 5, and deenergizing the first stage of the intermediate lenses 8. That is, the first stage of the intermediate lens is not excited. This method makes it possible to set the distance b to a large value of tens of millimeters. For the same voltage Vƒ applied to the filament 11 and for the same spacing I, the width D of the interference region D of the electron hologram can be increased.
In this way, a holography electron microscope adopts peculiar use conditions different from those of a normal TEM.
An imaging mode that is different from normal use conditions of a TEM as described above and intended to secure a wider interference region width is herein referred to as the intermediate magnification mode. On the other hand, an imaging mode in which the objective lens 5 is excited strongly and the focal distance is set to a small value is referred to as the high magnification mode. An imaging mode in which the objective lens 5 is excited very weakly or the auxiliary objective lens 6 is used to provide low magnifications of thousands of times is referred to as the low magnification mode. However, in the low magnification mode, high-resolution imaging owing to the objective lens 5 is sacrificed. Therefore, magnifications of thousands of times are not used in practical applications. There are three imaging modes, i.e., low, intermediate, and high magnification modes, in this way. The magnifications, interference fringe spacings, and interference region widths in these modes are given in the following table.
In the prior art holography electron microscope, the electron biprism 7 is placed between the objective lens 5 and the system of intermediate lenses 8 as shown in FIG. 7. Because the biprism 7 is mounted as an option or an accessory for the TEM, the biprism is positioned in a space that can accommodate it. This space lies between the objective lens 5 and the system of intermediate lenses 8.
As mentioned previously, the prior art holography electron microscope has three imaging modes (i.e., low, intermediate, and high magnification modes). It is to be noted that there is a blank magnification range between the low and intermediate magnification modes as can be seen from Table 1 above. However, there is no blank magnification range between the intermediate and high magnification modes and so the magnification can be varied continuously.
Therefore, with the prior art holography electron microscope, magnifications from about tens of thousands of times to 100,000xc3x97 cannot be accomplished. In terms of the interference region width, modes of hundreds of nanometers cannot be achieved.
Furthermore, even at an achievable magnification, the magnification mode must be selected according to the size of the object to be observed. In consequence, the magnification must be set meticulously taking account of even the resolution of the observation/recording device 10.
In this way, in the prior art holography electron microscope, limitations are imposed on the magnification. Also, much care must be exercised in the use of the instrument. Hence, electron holography based on transmission electron microscopy has not been a research tool that can be handled easily by anyone.
The present invention is intended to solve the foregoing problems. It is an object of the present invention to provide a holography electron microscope permitting one to easily perform electron holography without limitations on the magnification.
A holography electron microscope built in accordance with the present invention and solving the foregoing problems has an electron biprism located between a system of intermediate lenses and a system of projector lenses. Lenses, such as the intermediate lenses, located closer to the electron gun than the electron biprism permits one to set the magnification at will.
Other objects and features of the invention will appear in the course of the description thereof, which follows.